It has been long appreciated that if the magnetic field around a: field generating element, for example a generating coil, can be accurately mapped then it might be possible to determine the location of a field sensor, for example a sensing coil, relative to the generating coil, from the signals sensed by such a sensing coil. However, a problem associated with doing this is that there are in general many locations and/or orientations of the sensing coil within the field of the generating coil that will provide the same characteristic sensing signals in the sensing coil. In order to use a magnetic field for this purpose, additional information must therefore be provided.
Prior art approaches to providing the additional information required comprise either moving the generating and sensing coils relative to each other, or scanning the axis of the generated field past the sensing coil.
An example of the first approach is taught in U.S. Pat. No. 3,644,825 wherein a system is disclosed for locating the position of a field sensor, comprising two orthogonal sensing coils, relative to a field generating element which relies on having knowledge of the direction of motion of the sensor relative to the generator. It should be noted that this system cannot detect the location of an object unless there is such relative motion, and its direction is known.
The second approach of scanning the axis of the generated field is disclosed, for position location in two dimensions, in U.S. Pat. No. 3,121,228 and for position location in three dimensions in U.S. Pat. No. 3,868,565.
U.S. Pat. No. 3,121,228 describes how the distance and direction of a sensor, again comprising two orthogonal sensing coils, relative to a field generator, also comprising two orthogonal coils, can be determined. The two orthogonal generating coils are driven in phase quadrature so that the axis of the resultant field is caused to rotate within a plane. If the sensor is located within this plane then the axis of the field is guaranteed to scan past the sensor, and, because at any given distance from a field generator the field strength will be a maximum at the field axis, the sensor will detect a maximum in field strength at this time. The voltage induced in any one of the two coils forming the sensor will be dependent on the orientation of the coil relative to the field generator, and it is for this reason that in '228 two orthogonal coils are utilised in the sensor. The sum of these two voltages gives an indication of the distance between the sensor and generator, while the phase difference between the two voltages gives an indication of the direction of the generator relative to the sensor. It is thus essential to the operation of the location system of '228 that the axis of the field rotates and that two coils are present in the sensor.
In U.S. Pat. No. 3,868,555 this approach of scanning the axis, or maximum intensity vector, of the field past the sensor is extended to allow location of the sensor in three dimensions. Whereas in two dimensions it is sufficient merely to rotate the axis of the field within the plane to be sensed to guarantee it passing through the sensor, in three dimensions the axis would have to be rotated so that it described the surface of a sphere in order to be certain it encountered the sensor. To ensure that the axis passed through all points on the surface of a sphere the motion of the axis would be such that it encountered the sensor only very infrequently, and thus measurements by the sensor of the maximum field strength would also be infrequent. To avoid this the location system of '565 drives the Generator coils in a complex fashion so that the field axis tracks and rotates around the position of the sensor.
In order to locate the position of the sensor in three dimensions, according to the method of '565, three mutually orthogonal generating coils and three mutually orthogonal sensing coils are required and the three generating coils must be driven simultaneously by the three drive currents having amplitude and phase relationships between them which are controlled so as to direct the field axis towards the sensor.
The approach taken in '565 further requires that the various equations governing the voltage induced in a sensing coil located and orientated in a particular alternating magnetic field are solved dynamically in real time ie. during the acquisition of data from the sensing coil. This requirement, in addition to limiting the speed at which the sensor can move while still being located successfully by the system, also means that should it be desired to locate more than one sensor, all apparatus will need to be duplicated for each additional sensor.
According to a first aspect of the present invention there is provided a method of determining the location and the orientation of a field sensor relative to a plurality of field generators of know location, each field generator comprising a plurality of collocated field generating elements, the method comprising the steps of:                1) for each generator, energising each generating element and measuring the respective element at the field sensor,        2) for each field generator calculating, from the measurements of the field generated by each of its generating elements and an estimate of the orientation of the sensor, an estimate of the distance from that particular field generator to the sensor,        3) utilising the estimates of the distances from each of the field generators to the sensor, and the known location of the field generators to calculate the location of the sensor relative to the field generators,        4) employing the estimated location of the sensor from step 3) and the measurements of the field at the sensor to calculate a new estimate of the orientation of the sensor, and        5) repeating steps 2) to 4) iteratively, with step 2) employing the new estimate of sensor orientation from the preceding step 4), to improve the estimates of location and orientation of the sensor.        
The method of the first aspect of the present invention thus enables the location of a sensor to be determined without either relative motion between the sensor and the field generating element, or scanning of the axis of the field.
Since the method dissociates the stages of acquisition of data from the sensor, and processing of that data, rapid determination of the sensor location is facilitated. Furthermore the location of additional sensors may be determined simply by simultaneous measuring the field, generated by each generating element, at these other sensors and independently calculating their distances from the field generators. It should be noted that no modification of the field generating apparatus or method of driving the apparatus is required in order to determine the location of a plurality of sensors.
The applicants have discovered that advantageously the method of the first aspect of the present invention also allows the location of a sensor comprising a single sensing element, for example a sensing coil, to be determined, as will be explained subsequently. This is particularly advantageous for positioning applications in which two or more mutually orthogonal sensing coils, as required by prior art techniques, cannot be used.
According to a second aspect of the present invention there is provided a method of determining the location of a field sensor, comprising a plurality of collocated field sensing elements, relative to a field generator, comprising a plurality of collocated field generating elements, the method comprising the steps of:                1) energising a single field generating element to establish a field,        2) measuring a value of the field strength at the field sensor which is dependent on the location and orientation of the sensor within the field,        3) repeating steps 1) and 2) for each field generating element,        4) calculating, by utilising all the values measured in step 2 and an estimate of the direction of the sensor from the field generator, a direction dependent weighting factor for each field generating element so that the calculated field strength B is equal to the field strength B that would exist at the sensor if the axis of the field were directed towards the sensor,        5) iteratively altering the direction dependent weighting factors to maximise B and thus to determine to a desired level of accuracy the direction of the sensor from the field generator, and        6) employing the measured values of field strength to calculate the distance of the sensor from the field generator and have, from the direction of the sensor in step 5), the location of the sensor relative to the field generator.        
This aspect of the invention thus provides a method of locating a sensor relative to a single field generator.
The invention further provides apparatus suitable for carrying out the methods of the first two aspects of the invention.
In a first embodiment the invention enables a sensor comprising a single sensing coil to be located in three dimensions relative to a plane defined by three field generators.